Effects of N-ethylmaleimide on gonadotropin and beta-adrenergic receptor function coupled to rabbit luteal adenylyl cyclase.

The effects of the sulfhydryl-reactive alkylating agent N-ethylmaleimide (NEM) on the rabbit luteal adenylyl cyclase system were studied. Treatment of luteal membranes with NEM revealed three activities with differing sensitivities to NEM treatment. When luteal membranes were treated with NEM on ice for 30 min, it was found that NaF-stimulated adenylyl cyclase activity in cholate extracts of these membranes was most sensitive to this treatment. Half-maximal inhibition was obtained at 0.09 mM NEM. The activity of the stimulatory guanine nucleotide- and Mg-binding regulatory component (Ns), as assessed by functional reconstitution of NaF-stimulated adenylyl cyclase activity into membranes from the cyc-variant of the S49 mouse lymphoma, was less sensitive to this treatment, with half-maximal inhibition occurring at 0.69 mM NEM. In contrast, high affinity gonadotropin and beta-adrenergic binding, as assessed by competitive displacement of [125I]iodo-hCG by bovine LH and (-)3-[125I]iodocyanopindolol by isoproterenol, was unaffected by NEM concentrations up to 50 mM when membranes were treated on ice. However, when membranes were treated with NEM at 25 C for 30 min, high affinity gonadotropin and beta-adrenergic binding demonstrated similar sensitivities to NEM treatment, such that 50 mM NEM completely inhibited high affinity binding to both receptors. Under either of the conditions described above, neither the number of receptors nor the affinities of the labeled probes for their receptors were altered by NEM treatment. Thus, there appears to be at least three NEM-sensitive sites necessary for the functioning of the rabbit luteal adenylyl cyclase system, one associated with the catalytic component, one on Ns which interacts with the catalytic component, and one involved in high affinity agonist binding. Furthermore, it appears that formation of the high affinity binding state is regulated similarly for gonadotropin and beta-adrenergic receptors.